In many applications, such as drug discovery and development, environmental testing, and diagnostics, there is a need to analyze a large number of samples in an efficient and reproducible manner. Many of the techniques used to analyze fluidic samples require that the samples be tested in a serial manner. In such applications, the process of serial analysis can be automated through the use of a computer controlled robotics and automation. Such devices are generally called auto-injectors and are commonly interfaced to all manner of serial analysis systems including, but not limited to, chromatography systems, mass spectrometers, and spectroscopic detectors.
Typical auto-injectors include a plurality of sample reservoirs, a syringe or syringe-like sample transport system, and an injection valve along with the automation and computer control systems. Auto-injectors commonly mimic cumbersome manual injection methods in which a metered aliquot of a sample is aspirated from a desired sample reservoir into a transfer syringe. The aspiration process is often controlled by pulling back on a plunger or piston to create a negative pressure resulting in aspiration of the sample. The transfer syringe is then moved to and docked with a stationary injection valve. The sample aliquot is then transferred from the syringe to the injection valve by depressing the transfer syringe plunger or activating the piston. The sample fills an injection loop within the injection valve. Upon actuation of the valve the sample is introduced into the fluidic circuit and diverted to the analysis system.
The transfer syringe and the injection valve ports are then rinsed with an appropriate buffer or solvent to remove traces of the analyte to minimize contamination between samples. Contamination of the fluidic system with a sample can cause a significant barrier to the successful operation of a serial analysis system resulting in carryover and compromised data. After an appropriate cleaning protocol the entire process is repeated for the next sample. Various embodiments of this general approach to auto-injectors are available commercially. Sample reservoirs used in auto-injectors range from glass vials to 96 or 384-well microtiter plates. Sample reservoirs may be sealed with a plastic film or metal foil, or a septum. Some auto-injection devices use conventional syringes of various sizes attached to a robotic arm. Other devices use a tube attached to a small piston. The sample is aspirated into this tube and transferred to the injection valve. Some versions of auto-injectors attempt to increase throughput by using multiple syringes such that while an injection is being made by one syringe others are being washed. One auto-injector increases throughput with a simultaneous aspiration of eight samples. These samples are then loaded into the sample injection loops of eight separate injection valves. The samples are then sequentially diverted from each of the eight injection valves into the analysis system. Throughput is thus increased through the parallelism of the process, however at increased cost and complexity.
Mass spectrometry (MS) with atmospheric pressure ionization (API) is a commonly used technique for the analysis of complex mixtures. Variations of API-MS include electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). API-MS is used routinely in the pharmaceutical industry, environmental and forensic analysis, materials science, and in scientific applications. Both quantitative and qualitative information about specific compounds in complex mixtures can be obtained with the use of API-MS methods.
However, API-MS has several drawbacks. Traditionally, MS is a serial process in which samples are analyzed sequentially unlike parallel analysis schemes typically employed in many optical analysis systems. Sequential analysis can be impractical and in many cases economically unviable if very large numbers of samples are to be analyzed.
Furthermore, many compounds typically found at high concentrations in complex biological, chemical, or environmental samples, such as salts, buffers, ionic or non-ionic detergents, proteins or enzymes, and other cofactors can cause a significant reduction in the amount of target signal observed in mass spectrometry. Interference from high concentrations of non-volatile components are particularly troublesome because in addition to causing signal suppression non-volatile compounds tend to build up in the source region of the MS and gradually result in a decline in instrument performance.
The inherent expense involved in purchasing and operating mass spectrometers makes it highly desirable to improve productivity by devising methods and devices for increasing the analysis throughput (i.e. the number of samples that can be analyzed in a given time). Any method and device that attempts to increase throughput in API-MS must address several key issues such as: (1) a rapid system for delivery of a sample to the mass spectrometer must be designed; (2) the components of complex mixtures that cause suppression of the target signal must be isolated and removed from the analytes of interest; (3) the non-volatile components of complex mixtures that build up in the MS source and result in a decay of instrument performance over time must be isolated and removed; and (4) each sample must be cleaned from the analysis system to an acceptable level before the next sample is analyzed to prevent sample-to-sample carryover that will result in contamination of the data.
Liquid chromatography (LC) can be used to remove the salts, buffers, and other components from complex mixtures that may cause suppression of the MS signal of interest or result in degradation of MS instrument performance. Conventional liquid chromatography (LC) and its variations, such as high performance liquid chromatography (HPLC), typically involve flowing a liquid sample over a solid, insoluble matrix (generally referred to as Solid Phase Extraction (SPE)) commonly packed in a column format. The liquid sample includes an analyte(s) of interest that has an affinity for the matrix under certain conditions of pH, salt concentration, or solvent composition. Affinity of the analyte(s) of interest to the matrix may be due to hydrophobic or hydrophilic interactions, ionic interactions, molecular size, or coordination chemistry. In a highly specific variation, antibodies immobilized to the matrix are used to selectively capture molecules containing a highly specific epitope from complex mixtures.
As a result of the analyte(s) affinity to the matrix, the analyte(s) binds to the matrix and becomes immobilized while other (undesired) components of the liquid sample flow through the matrix and are removed. The analyte(s) of interest are then eluted away from the matrix by changing the conditions of the flowing liquid, such that the analyte of interest no longer has affinity for the matrix. For example, changes in pH, ionic strength, solvent composition, temperature, and/or other physicochemical parameters may weaken the affinity of the analyte(s) for the matrix.
However, the traditional use of liquid chromatography in high-throughput mass spectrometry has limitations. Very often, the throughput of a serial analysis is limited by the time it takes to collect the signal from an individual sample. In liquid chromatography applications, the matrix output signal from an analyte of interest is in the form of a peak, and the width of this peak in time is the ultimate determinant of the maximum throughput. A key factor in increasing mass spectrometry throughput is the elution of the samples of interest from the insoluble matrix as a tight, sharp band that is presented to the mass spectrometer in the shortest amount of time. For example, to achieve an overall throughput greater than 30 seconds per sample, with baseline resolution of each sample, the peak width must be narrower than 30 seconds. As throughput is increased, more stringent requirements on the peak width must be imposed. If the throughput begins to approach the peak width, the sequential samples begin to overlap, baseline resolution between samples in the MS is lost, and accurate quantification for each sample is no longer possible.
In traditional LC, the analyte(s) of interest that are bound to the insoluble solid matrix (typically packed in a column format) are eluted away from the matrix by changing various properties of the liquid flowing over the matrix such that the analyte(s) are no longer immobilized on the column. However, as the analyte(s) flow through the length of the matrix a phenomenon known as band broadening occurs, in which linear diffusion causes the volume which contains the focused analyte(s) to expand. Consequently, the concentration of the analyte of interest presented to the mass spectrometer (or other analyzer) is decreased, and a broad peak is produced that makes High Throughput Screening (HTS) problematic.